Licensed Assisted Access (LAA) is a technology considered for inclusion in LTE (Long Term Evolution) Release 13 to meet the ever increasing demand for high data rate in wireless cellular networks by utilizing the carrier aggregation (CA) feature supported in LTE-A (LTE Advanced) systems to combine the data transmission over a licensed primary carrier and one or more unlicensed component carriers. The 5 GHz band is of current interest in 3GPP (the Third Generation Partnership Project). For fair coexistence with the incumbent systems at the 5 GHz band such as IEEE 802.11 (Institute of Electrical and Electronics Engineers standard 802.11) based wireless local area networks (WLAN), Listen-Before-Talk (LBT) is a feature recommended for inclusion in Release 13 LAA system.
According to 3GPP TS (Technical Specification) 36.213, for unlicensed band transmission, the category 4 LBT mechanism is used for LAA DL transmission bursts containing PDSCH (physical downlink shared channel). With category 4 LBT, the LAA eNB (Evolved NodeB) can sense the channel and perform DL (downlink) transmission at any time instant. However, conventionally, the starting of the transmission for DL burst is aligned with the Pcell (Primary cell) subframe boundary, as the existing Release-12 CA mechanism in LTE assumes Pcell-aligned transmission on the Scell (Secondary cell). If such a Pcell-aligned restriction is enforced, the interval from the ending of the LBT until the Pcell subframe boundary can be wasted, as it would not be utilized for data transmission. In this regard, a partial TTI (transmission time interval) is defined on a subset of OFDM (orthogonal frequency division multiplexing) symbols within a DL subframe, while still maintaining the Pcell-aligned timing relationship for the DL burst transmission.
With a partial subframe, even though it is possible for the eNB to transmit immediately after completing the LBT, the eNB may start PDSCH transmission at certain known OFDM symbol positions within a subframe with respect to the Pcell subframe boundary to limit the UE (user equipment) blind detection complexity in determining the starting positions of the DL transmission burst. Limiting the starting positions within the subframe may also help reduce eNB scheduling complexity, as eNB may a priori prepare partial subframes for all possible starting positions.